1. Field of the Invention
This invention relates to a dual phase stainless steel suitable for use in sour wells, and more particularly to a dual phase stainless steel capable of exhibiting excellent corrosion resistance in Cl.sup.- --CO.sub.2 --H.sub.2 S environments and thus suitable for use in sour wells.
2. Description of the Prior Art
Reflecting the scarcity of petroleum resources, deep wells and sour wells which have hitherto been disregarded have been recently given a second look. However, these wells contain lots of chlorides together with CO.sub.2 and H.sub.2 S in many instances. Accordingly, conventional carbon steel and low alloy steels cannot be used as tubing materials for such wells due to their considerably strong corrosiveness and there is an outstanding demand for high alloy materials having excellent corrosion resistance.
As such high alloy materials, may be mentioned 13Cr martensitic stainless steels, dual phase stainless steels of austenite and ferrite, high nickel austenitic alloys, nickel-base alloys, cobalt-base alloys, titanium-base alloys and the like. Among these alloy materials, a high Cr dual phase austenite/ferrite stainless steel is excellent in general corrosion resistance to CO.sub.2 and also resistant against stress corrosion cracking due to chlorides, and has found a wide-spread commercial utility as a tubing material for corrosive wells.
However, the greatest problems of such dual phase stainless steel reside in that it is susceptible of developing sulfide stress corrosion cracking(SSCC) where chlorides and a lot of H.sub.2 S are concommitantly present and it involves a danger of developing crevice corrosion and/or pitting corrosion in the concommitant presence of chlorides and CO.sub.2. Thus, the development of a dual phase stainless steel durable against these corrosions has been waited for.
Incidentally, the following three characteristics are most important among characteristics generally required for tubing materials to be used in sour wells:
(1) Not to develop stress corrosion cracking in Cl.sup.- --CO.sub.2 --H.sub.2 S environments;
(2) Resistant to localized corrosions such as crevice and pitting corrosions in Cl.sup.- --CO.sub.2 --H.sub.2 S environments; and
(3) To have high mechanical strength.
Of the above important characteristics, it is possible to meet the third requirement for high mechanical strength by cold-working dual phase stainless steel. However, the stress corroding cracking problem referred to in the first requirement becomes extremely easy to occur under such cold-working conditions. Thus, it is desirable to have high mechanical strength under a solution heat treated condition (i.e., annealed).